3 Answers
3

The original meaning of enormity is something which departs from normal standards, in other words something that is ABnormal. Its current, and rather specialised, use is to describe something that is some kind of gross transgression, often in a legal context. However, perhaps because of its similarity to enormous, it started to be used over 200 years ago to mean something very large but, in the words of the Oxford English Dictionary, ‘the use is now regarded as incorrect’. That meaning can perhaps be defended etymologically, but, as you suggest, few people would use it in any kind of figurative sense.

I believe it happens because people confuse the word enormous with enormity. As you know, enormity has the synonyms of atrocity, horribleness, and abomination. There is nothing about physical size in its definition.

I like the answers already provided, but it appears that the confusion JLG alludes to is pervasive enough to prompt a shift in meaning (or, at least, a dispute about a shift in meaning). In addition to Barrie's footnote from the OED, I found some interesting tidbits in several online dictionaries:

For example, Dictionary.com lists a tertiary meaning that alludes to size:

enormity:
1. outrageous or heinous character; atrociousness: the enormity of war crimes.
2. something outrageous or heinous, as an offense: The bombing of the defenseless population was an enormity beyond belief.
3. greatness of size, scope, extent, or influence; immensity: The enormity of such an act of generosity is staggering.

The Macmillan Dictionary lists "of large size" as the primary meaning, but includes a note that some "consider this to be incorrect:"

enormity: 1 used to mean the extremely large size of something, although some people consider this to be incorrect: He considered the
enormity of the task he had been given. 2 the fact that something is
morally wrong, or the degree to which it is morally wrong: They were
desperate to conceal the enormity of what they had done. 2a. an
action that is morally wrong.

Merriam-Webster contains a rather extensive usage discussion concerning its third meaning of enormity:

enormity 1 : an outrageous, improper, vicious, or immoral act <the enormities of state power — Susan Sontag> <other enormities
too juvenile to mention — Richard Freedman> 2 : the quality or state
of being immoderate, monstrous, or outrageous; especially : great
wickedness <the enormity of the crimes committed during the Third
Reich — G. A. Craig> 3 : the quality or state of being huge :
immensity <the inconceivable enormity of the universe> 4 : a quality
of momentous importance or impact <the enormity of the decision>

Usage Discussion of ENORMITY

Enormity, some people insist, is improperly used to denote large size.
They insist on enormousness for this meaning, and would limit enormity
to the meaning “great wickedness.” Those who urge such a limitation
may not recognize the subtlety with which enormity is actually used.
It regularly denotes a considerable departure from the expected or
normal (they awakened; they sat up; and then the enormity of their
situation burst upon them. “How did the fire start?” — John
Steinbeck). When used to denote large size, either literal or
figurative, it usually suggests something so large as to seem
overwhelming (no intermediate zone of study. Either the enormity of
the desert or the sight of a tiny flower — Paul Theroux; the
enormity of the task of teachers in slum schools — J. B. Conant) and
may even be used to suggest both great size and deviation from
morality (the enormity of existing stockpiles of atomic weapons —
New Republic). It can also emphasize the momentousness of what has
happened (the sombre enormity of the Russian Revolution — George
Steiner) or of its consequences (perceived as no one in the family
could the enormity of the misfortune — E. L. Doctorow).

In any case, whichever side of the debate you support, using the phrase the enormity of his achievements would be ripe for misinterpretation, particularly if you were alluding to something positive.